More on Asteroid Impacts

From the Register (U.K.):

Between 2000 and 2013, the Earth was hit by 26 asteroids that exploded with a force of between one and 600 kilotons – an average of one every six months. Even more concerning is that in all cases the asteroids themselves weren't detected in space and only came to light when they detonated in Earth's atmosphere.

The study was carried out by the B612 Foundation, a group set up by three former astronauts who are worried about the threat of asteroids to life on Earth. The foundation's CEO (and former shuttle pilot) Dr. Ed Lu presented the report's findings at a press conference in Seattle's Museum of Flight on Tuesday.

"While most large asteroids with the potential to destroy an entire country or continent have been detected, less than 10,000 of the more than a million dangerous asteroids with the potential to destroy an entire major metropolitan area have been found by all existing space or terrestrially operated observatories," said Lu.

"Because we don't know where or when the next major impact will occur, the only thing preventing a catastrophe from a 'city-killer' sized asteroid has been blind luck," he concluded.

The study notes that four of this century's collisions have been larger than the atomic bombs that took out Hiroshima and Nagasaki. In 2013, over a thousand people were injured when an asteroid exploded over Chelyabinsk, and 20 kiloton impacts were recorded over Indonesia, the Southern Ocean, and the Mediterranean.

All of these are dwarfed by the 1908 Tunguska impact, when the earth wandered into the path of a comet or large asteroid that exploded with a force of around 10 megatons – an explosion that leveled the surrounding forests and blasted down trees for 2,150 square kilometers (830 sq miles.)

 Also, a BBC article that explains more about the B612 group and their proposed telescope.